1. Field of the Invention
This invention concerns a medium for recording and reproducing electric signals.
2. Description of the Prior Art
In recent years, large capacity external memories have become increasingly important with the increase in the amount of information. In addition to magnetic tapes and floppy discs existent so far, hard magnetic discs or optical discs have been highlighted as the external memory means. Among all, the optical discs are expected to play an important role as memory means in the future and vigorous studies have been made both for read-only discs and for rewritable discs.
The optical disc utilizing laser beams (hereinafter simply referred to as the optical disc) is a memory medium having such a great capacity that can store information of documents amounting to several tens of thousands of A-4 size sheets on a single disc with a diameter as small as 30 cm or less. Accordingly, each bit formed therein is as small as 1 .mu.m in size. Further, since writing and reading of the information are performed by focusing the laser beam restricted to about 1 .mu.m diameter on the disc rotating at a high speed under positional detection and focal adjustment, even extremely small scurs, obstacles, as well as deformations, warps and deviations in the substrate may cause signal errors. Therefore, the utmost care is required for the production of optical discs comparable to the same care required for the semiconductors. Further careful protecting structures have been adopted for most of the optical discs in order to improve the reliability. The protecting structures can generally be classified into two types, that is, a so-called air sandwich structure in which two disc sheets are appended with each other including an air layer therebetween and a closely bond structure in which adhesives are coated on the entire surface of a recording layer. Among them, the production steps are complicated, the thickness of the disc sheet is increased and the structure tends to be easily deformed in the former. While on the other hand, the latter close bond structure has no such drawbacks and the shape of the completed disc is stable. Example of the close bond type can include those in which discs of an identical shape are appended by using hot melt adhesives with the signal recording surfaces being on the inner side or in which a protecting layer of an acrylic lacquer is formed on a signal recording layer (Japanese Patent Application of Laid-Open No. 54-89704). The optical discs of these protecting structures are advantageous in view of their high productivity with a small amount of deformities in the disks, but their weatherproofness is not yet satisfactory. Those using the hot melt adhesives gradually lose the bonding strength, particularly, under high temperature and high humid conditions and the bonded portion may possibly be defoliated. The acrylic lacquer also lacks in the long time reliability in that the metallic recording layer rapidly causes remarkable corrosion aging, etc.